Transport of lysosomes decreases in the perinuclear region: Insights from changepoint analysis

Lysosomal Repair in Health and Disease

Lysosomes are essential organelles degrading a wide range of substrates, maintaining cellular homeostasis, and regulating cell growth through nutrient and metabolic signaling. A key vulnerability of lysosomes is their membrane permeabilization (LMP), a process tightly linked to diseases including aging, neurodegeneration, lysosomal storage disorders, and cardiovascular disease. Research progress in the past few years has greatly improved our understanding of lysosomal repair mechanisms. Upon LMP, cells activate multiple membrane remodeling processes to restore lysosomal integrity, such as membrane invagination, tubulation, lipid patching, and membrane stabilization. These repair pathways are critical in preserving cellular stress tolerance and preventing deleterious inflammation and cell death triggered by lysosomal damage. This review focuses on the expanding mechanistic insights of lysosomal repair, highlighting its crucial role in maintaining cellular health and the implications for disease pathogenesis and therapeutic strategies.

DNA tensiometer reveals catch-bond detachment kinetics of kinesin-1, -2 and -3

Bidirectional cargo transport by kinesin and dynein is essential for cell viability and defects are linked to neurodegenerative diseases. Computational modeling suggests that the load-dependent off-rate is the strongest determinant of which motor 'wins' a kinesin-dynein tug-of-war, and optical tweezer experiments find that the load-dependent detachment sensitivity of transport kinesins is kinesin-3 > kinesin-2 > kinesin-1. However, in reconstituted kinesin-dynein pairs vitro, all three kinesin families compete nearly equally well against dynein. Modeling and experiments have confirmed that vertical forces inherent to the large trapping beads enhance kinesin-1 dissociation rates. In vivo, vertical forces are expected to range from negligible to dominant, depending on cargo and microtubule geometries. To investigate the detachment and reattachment kinetics of kinesin-1, 2 and 3 motors against loads oriented parallel to the microtubule, we created a DNA tensiometer comprising a DNA entropic spring attached to the microtubule on one end and a motor on the other. Kinesin dissociation rates at stall were slower than detachment rates during unloaded runs, and the complex reattachment kinetics were consistent with a weakly-bound 'slip' state preceding detachment. Kinesin-3 behaviors under load suggested that long KIF1A run lengths result from the concatenation of multiple short runs connected by diffusive episodes. Stochastic simulations were able to recapitulate the load-dependent detachment and reattachment kinetics for all three motors and provide direct comparison of key transition rates between families. These results provide insight into how kinesin-1, -2 and -3 families transport cargo in complex cellular geometries and compete against dynein during bidirectional transport.

Endolysosomal vesicles at the center of B cell activation

The endolysosomal system specializes in degrading cellular components and is crucial to maintaining homeostasis and adapting rapidly to metabolic and environmental cues. Cells of the immune system exploit this network to process antigens or promote cell death by secreting lysosome-related vesicles. In B lymphocytes, lysosomes are harnessed to facilitate the extraction of antigens and to promote their processing into peptides for presentation to T cells, critical steps to mount protective high-affinity antibody responses. Intriguingly, lysosomal vesicles are now considered important signaling units within cells and also display secretory functions by releasing their content to the extracellular space. In this review, we focus on how B cells use pathways involved in the intracellular trafficking, secretion, and function of endolysosomes to promote adaptive immune responses. A basic understanding of such mechanisms poses an interesting frontier for the development of therapeutic strategies in the context of cancer and autoimmune diseases.

Palmitate-mediated disruption of the endoplasmic reticulum decreases intracellular vesicle motility

Essential cellular processes such as metabolism, protein synthesis, and autophagy require the intracellular transport of membrane-bound vesicles. The importance of the cytoskeleton and associated molecular motors for transport is well documented. Recent research has suggested that the endoplasmic reticulum (ER) may also play a role in vesicle transport through a tethering of vesicles to the ER. We use single-particle tracking fluorescence microscopy and a Bayesian change-point algorithm to characterize vesicle motility in response to the disruption of the ER, actin, and microtubules. This high-throughput change-point algorithm allows us to efficiently analyze thousands of trajectory segments. We find that palmitate-mediated disruption of the ER leads to a significant decrease in vesicle motility. A comparison with the disruption of actin and microtubules shows that disruption of the ER has a significant impact on vesicle motility, greater than the disruption of actin. Vesicle motility was dependent on cellular region, with greater motility in the cell periphery than the perinuclear region, possibly due to regional differences in actin and the ER. Overall, these results suggest that the ER is an important factor in vesicle transport.

Dynamic Actin Filament Traps Mediate Active Diffusion of Vesicular Stomatitis Virus Ribonucleoproteins

A recently developed variational Bayesian analysis using pattern recognition and machine learning of single viral ribonucleoprotein (RNP) particle tracks in the cytoplasm of living cells provides a quantitative molecular explanation for active diffusion, a concept previously "explained" largely by hypothetical models based on indirect analyses such as continuum microrheology. Machine learning shows that vesicular stomatitis virus (VSV) RNP particles are temporarily confined to dynamic traps or pores made up of cytoskeletal elements. Active diffusion occurs when the particles escape from one trap to a nearby trap. In this paper, we demonstrate that actin filament disruption increased RNP mobility by increasing trap size. Inhibition of nonmuscle myosin II ATPase decreased mobility by decreasing trap size. Trap sizes were observed to fluctuate with time, dependent on nonmuscle myosin II activity. This model for active diffusion is likely to account for the dominant motion of other viral and cellular elements. IMPORTANCE RNA virus ribonucleoproteins (RNPs) are too large to freely diffuse in the host cytoplasm, yet their dominant motions consist of movements in random directions that resemble diffusion. We show that vesicular stomatitis virus (VSV) RNPs overcome limitations on diffusion in the host cytoplasm by hopping between traps formed in part by actin filaments and that these traps expand and contract by nonmuscle myosin II ATPase activity. ATP-dependent random motion of cellular particles has been termed "active diffusion." Thus, these mechanisms are applicable to active diffusion of other cellular and viral elements.